A semiconductor element including a silicon carbide (SiC) semiconductor as a base material has attracted attention as a device capable of achieving a high breakdown voltage and low loss. In particular, a field-effect transistor (metal-oxide semiconductor field effect transistor; MOSFET) with a metal-insulator-semiconductor junction has been requested to achieve higher reliability in terms of its application to power electronics.
As an example, if such a semiconductor element is applied to an inverter circuit and the like and an inductive load or a resistive load operates therein, a high voltage as a power source voltage may be applied to a drain of the element in an ON condition as a result of the occurrence of a load short-circuit such as an arm short-circuit. In this case, a high-current may flow into this element. This condition induces a drain current of several times to several tens of times higher than a rated current. This leads to breakdown of the element if the element does not have an appropriate protective function.
In order to obviate the element breakdown, an excessive drain current (overcurrent) should be interrupted by detecting this drain current and inputting an OFF signal to a gate electrode in response to the detection before the element breakdown occurs. Thus, the element is required to have robustness that does not cause the element breakdown over a period of time or longer than this period from when the overcurrent is detected at the occurrence for example of a load short-circuit until when the OFF signal is input to the gate electrode. Specifically, the element is strongly desired to have a high short-circuit tolerance. The short-circuit tolerance is determined as a period of time from when a load short-circuit or the like occurs until when element breakdown occurs.
Patent literature 1 discloses a technique of enhancing the short-circuit tolerance of an IGBT (insulated gate bipolar transistor) as a typical power device. The IGBT of patent literature 1 has a structure where an emitter layer to become a path for an ON current includes a high-resistance emitter layer (high-resistance region) and a low-resistance emitter layer (low-resistance region) that are connected in parallel to each other between an emitter electrode and a channel region. In this structure, the occurrence of a load short-circuit causes flow of an electron current in the emitter layer to increase voltage drop. This reduces a saturation current value to enhance the short-circuit tolerance. The low-resistance region reduces a contact resistance between the emitter electrode and the emitter layer, thereby achieving a low ON voltage.